Since the mid-to-late-1980s, intra-arterial stents have found extensive use as a treatment to prevent restenosis subsequent to balloon angioplasty or atherectomy. A recurrent problem is that excessive tissue growth (intimal hyperplasia) at the site of the balloon dilation or atherectomy plaque excision results in restenosis of the artery. One possible solution to this problem is to coat the stent with an anti-thrombogenic surface so as to reduce platelet and fibrin deposition. This is described in U.S. Pat. No. 4,768,507 issued Sep. 1988, to Robert E. Fischell and Tim A. Fischell entitled "Intravascular Stent and Percutaneous Insertion Catheter System for the Dilation of an Arterial Stenosis and the Prevention of Restenosis" which is incorporated herein by reference. Although an anti-thrombogenic coating can prevent acute thrombotic arterial closure and decrease the need for anticoagulant drug therapy, there is still an urgent need to decrease restenosis which is caused by intimal hyperplasia. Similarly, stents have been used in the bile duct of patients with liver cancer to maintain duct patency. However, malignant cells soon grow through the stent and close the bile duct or severely impede its function.